Adjustment of magnetic tachometers



Dec. 14,1926. 1,611,089

c. T. WALLIS ADJUSTMENT OF MAGNETIC TACHOMETEHS Filed Sept.` 24, 1923Fung. Il.,

5\ /7 20- @l 6^- l mesmm Hmvcelmmm C Vl/@MA LM om@ /7 Patented Dec. 14,1926.

UNITED STATES PATENT OFFICE.

CYRIL T. WALLIS, OF ROCHESTER, NEW YORK, ASSIGNOR TO NORTH EAST ELECTRICCOMPANY, OF ROCHESTER, NEW YORK. A CORPORATION OF NEW YORK.

ADJUSTMENT OF MAGNETIC TACHOMETERS.

Application filed September 24, 192B. Serial No. 664,517.

This invention relates to the adjustment of tachometers of the magnetictype, in which a magnetic field, produced by a permanent magnet, iseffective on an indicator by reason of eddy currents therein resultingtrom relat-ive movement of the field and the indicator.

In instruments of the type in .question the correctness of theindications depends,

among other factors, upon the strength ot' the magnetic field, andaccordingly it has been customary to provide, in the construction ofsuch instruments, some means for varying this strength, either byshunting a 5 variable portion of the magnetic lines of torce or bychanging the relative positions of the parts, so that the instrument maybe calibrated or adjusted whenever necessary.

The object ot the present invention is to 3" simplify the constructionand the adjustment of magnetic tachometers by eliminating the necessityoi the provision and the use of the adjusting means just referred to. Tothis end I have devised a method, and means for practising it, by whichnecessary changes in the strength ot the magnetic iield of such aninstrument may be made by changing the density of magnetization of thepermanent magnet, and whereby this may be accomplished by means externalto the instrument, while the instrument is in operation.

In the accompanying drawings Fig. 1 is a partly diagrammaticfront-elevation of apparatus embodying the present invention, togetherwith a magnetic tachometer in position to be adjusted; and Fig. 2 is asec- -tional side-elevation showing the relative position of thetachometer and of the electromagnet of the apparatus. In the apparatusillustrated in the drawings the tachometer 5, which is to be calibrated,is of the type commonly used on motor-vehicles and described as aspeedometer. It is mounted on a supporting plate 6, which also carries amaster speedometer or tachometer 7 of any preferred type. The twoinstruments are driven by flexible shafts enclosed in the usual tubularcasings 8 and 9, and the two shafts are i 5") connected, through gears10, with a pinion 11 which is fixed on the shaft of an electric motor12.

The motor is arranged to be driven at variable speed, and for thispurpose it is connected, with a suitable source 13 of electricity,through a rheostatlt.

The adjustment of the speedometer is effected through theinstrumentality of an electromagnet comprising two parallel cores 15provided with windings 16 and connected and supported by a yoke-plate17. This yoke-plate is Vpivoted between arms 18 at the upper end of acolumn 19, so that the poles 20 of the magnet may be swung towards andfrom the front of the speedometer, and the arms 18 are also pivoted tothe column for the purpose of adjustment. The pivots 34 connectingthesev several arts are provided with springs to cause frictionalretention of the parts in any adjusted position.

. The electromagnet is arranged to be energized with variable strengthand reversible polarity, for longer or shorter periods as required. Forthis purpose it is con nected with the source of electricity, 13,through wires 21 and 22, a rheostat 23, a circuit-closing key 24 and apole-changing switch or commutator 25.

The method of adjustment is applicable to magnetic tachometers ofvarious construction, but is illustrated as used in connection with aninstrument having a ringshaped magnet 26 which is mounted in anon-magnetic cup 27 at the upper end of a rotary shaft 28. This magnetcooperates with an oscillatory indicator comprising outer and innerskirts or flanges 29 and 30, carried by an arm 31 which is fixed to avertical pivot 32. The pivot is connected with a spring 33 by which themovement of the indicator is controlled. The operation' of theinstrument need not be described herein, as it is of a well known type.

The speedometer may be calibrated either complete .or with the frontglass and taceplate removed, but the latter method is preferable as itpermits the electromagnet to be brought closer to the permanent magnet26 of the instrument. j

With the parts in the position shown in the drawings, the operatorstarts the electric motor and adjusts its speed to cause the masterspeedometer to indicate some convenient speed of operation. If thespeedometer to be calibrated is inaccurate it will give an indicationdiierent from that of the master. The operator then sets 'the rheostat23 to minimum-current position, moves the commutator 25 to one positionor the other, according as the strength of the magnet 26 is to beincreased or diminished, and then taps the key 24 one or more times soas momentarily to energize the electromagnet. As the speedometer 5 isconstantly running during this operation it at once gives an indicationof any change which has been produced in its magnet. If therequired'adjustment is not at once effected the operator continues -totap the key 24, and at the same time moves the rheostat 23 gradually inthe direction to increase the strengthof they current owing through theelectromag'net, until the reading of the speedometer 5 indicates, by itscoincidence with that of the master, that the required correction hasbeen accomplished. j

If the effect at first produced happens t0 be in the Wrono' sense. orhas been carried magnet'izing and demagnetizing ring-magnets disclosedin my application for Letters Patent of the United States tiled Oct.'13, 1922, Serial N o. 594,415, since the eect of the electromagnet is toproduce a magnetic field, of an intensity sufficient to overcome thecoercive force of the speedometer magnet, thisield traversing a limitedpart of the length of the latter, and the rotation of the latter causingall parts of it to be successively passed through the i'eld. Theinvention is not limited to use with an instrument of this particularconstruction, however, since the electromagnet, by suitablemodifications in'its size and strength,

and in the position in which it is applied to tbe tachometer, may becaused, in any case, to produce a eld eflectivc upon the permanentmagnet of the instrument and of suiicient intensity ,to overcome thecoercive force of the permanent magnet.

The invention claimed is:

1; Apparatus, for adjusting a magnetic tachometer,` comprising, incombination, means for supporting a 'tachometer; means for actuating thetachometer at different known speeds; means for simultaneously4producing a magnetic field operative upon the magnet of the tachometerwith a strength sullicientto overcome the coercive force of the magnet;and means-for varying the eiectiveness of the field according to theindications of the tachometer.

2. Apparatus, for adjusting a magnetic tachometer, comprising, incombination, means for supporting a tachometer; means for actuating thetachometer at a known' speed; an electromagnet; means for support# ingthe electromagnet with its cles in proximity to the magnet of the taciometer; and a rheostat and a commutator connected with theelectromagnet. v j

Apparatus, for adjusting a magnetic tachometer, comprising, incombination, a speed-indicator; an electric motor; drivingconnectionsthrough which thev motor may drive the speed-indicator and thetachometer simultaneously; means for varying the speed of the motor; anelectromagnet; means; for supporting the magnet in a position such thatits lield is effective upon the magnet ot the tachometer while thetachometer ie actuated by the motor; and means for controlling theenergization of the electromagnet to control its eiect on the magnet ofthe tachometer.

CYRIL T. WALLIS.

